FX Read Noise Comparison
Prepared 2012-01-11 by Bill Claff (last revised 2012-02-16)

Read Noise values for Nikon FX DSLRs were converted to Engineering Dynamic Range (DR) for the purpose of comparison.
White Value was not taken into consideration. The effect on DR when White Value is known is negligible and pretty consistent across camera models.
With the exception of the D3X Read Noise values were determined from the Optical Black data in NEF files.

The D4 still appears to be an improvement over the previous FX bodies and the D800 also appears to be a strong performer.

Values are not shown for ISOs that are achieved through digital gain.

Note that the crossover from analog to digital gain is quite low for the D800.
Values lower than ISO 400 for the D800 are not known; try to resist guessing at them!
(The dip between ISO 100 and ISO 200 for the D4 bears further investigation.)

This following chart shows the raw DR values:

The x-axis is ISO in EV_S which is log₂(ISO/3.125). Note that ISO 100 is 5.0 EV_S.
Dotted lines were added to some data sets for clarity.
More Nikon D800 data points are needed, below ISO 400 and between ISO 800 and ISO 1600.

The following chart shows these same values as adjusted for photosite site:

I think these normalized values make for a fairer comparison.

It looks like the new Nikon D4 is the clear winner as far as Read Noise is concerned.

With limited initial data, the D800 looks very good.